Structural modification of eucalypt pulp lignin in a totally chlorine free bleaching sequence including a laccase-mediator stage

Abstract

Modification of eucalypt pulp residual lignin in a totally chlorine free (TCF) bleaching sequence including a laccase-mediator stage, in combination with previous oxygen delignification and subsequent peroxide bleaching, was investigated and compared with a chemical TCF sequence. The enzymatic treatment resulted in a strong oxidative modification, as shown by lignin unit 13C2,6-1H2,6 correlations in HSQC 2D-NMR spectra and presence of oxidized pyrolysis markers. The frequency of inter-unit linkages in residual lignin was also modified, as shown by different side-chain 13C-1H correlations. Moreover, H2,6 correlations with side-chain carbons in HMBC spectra revealed the presence of Cα keto and carboxyl groups in lignin from the oxygen-delignified and especially from the enzymatically-treated pulp, together with normal (Cα-hydroxylated) lignin units. Moreover, the results obtained showed that most of this oxidatively-altered lignin is removed under the alkaline conditions used in the peroxide stage. In this way, the pulp kappa number decreased, and the final residual lignin was more structurally-related to that found before the oxidative stages (although with lower content of resinol structures, and higher presence of aryl-aryl ether structures and carboxyl-bearing terminal units). However, the action of hydrogen peroxide is required to attain the high brightness, over 91% ISO, found in final pulp after the enzyme-containing TCF sequence.